1. Field of the Invention
This invention relates to a heat hardenable coating composition based on polyurethane urea, in particular for producing coatings which are permeable to water vapor on substrates and the coated substrates produced therefrom.
2. Description of the Prior Art
Coatings which are permeable to water vapor may be produced by various means. One process which has been known for a long time enables micropores to be produced in a coating by physical means. In these processes a polymer (this is to be understood hereinafter to denote mainly a polyurethane) dissolved in a solvent is applied as a layer to a substrate or a separating support and passed in the wet state through a bath which contains a non-solvent for the polymer which is miscible with the solvent. The non-solvent therefore penetrates the layer of solution where it gradually precipitates and coagulates the solid substance. When the resulting solidified film dries, both the solvent which escapes and the non-solvent leave micro channels behind which are available for the transport of water vapor in the coating.
By the addition of salt powders to polymer solutions it is possible in a similar manner to produce coatings from which the salt may be washed out with water, leaving microcavities behind.
The perforation of non-porous films by means of high energy electron beams also results in films with high water vapor permeability which can be laminated. (In the coating obtained which is permeable to water vapor, the volume of water vapor transported per unit time and surface area is generally 10 times as great as in a conventional non-porous film.)
These processes generally have the disadvantage of being chemically complicated or requiring elaborate apparatus or giving rise to considerable problems of waste.
A variation of the microporous coating process which does not require the complicated immersion bath technology has recently been carried out but has led to substantially the same results. In this process, also known as evaporation coagulation, sufficient water is added to the solution of the polymer in a low boiling solvent to ensure that the resulting spread coating paste is only just still stable and can be applied.
During the drying process the organic solvent is the first to be removed and the increasing proportion of water then precipitates the solid substances as in the immersion process and finally evaporates to leave a microporous structure in the film.
All these processes have the major disadvantage that microchannels or microcavities weaken the permeable coatings. This means that the mechanical strength and abrasion resistance are considerably inferior to those of a non-porous film. Accordingly, it is an object of the present invention to produce coatings which overcome these deficiencies.
There have been many attempts to obtain permeability to water vapor in a coating not only by physical methods but also by chemical means. Attempts have been made to produce such coatings with polyurethanes based on water-soluble or hydrophilic starting components. DE-AS Nos. 1,220,384 and 1,226,071 describe polyurethanes which are prepared from glycols, diisocyanates and a difunctional hydrophilic starting component based on a macrodiol which renders the coating permeable to water vapor. Both of these applications deal with a polyethylene glycol having a molecular weight of about 1000 and differ from one another only in the "vulcanization" mechanism, i.e., the subsequent cross-linking of the polyurethane elastomer.
Compact top coats on composite materials of textile substrates and microporous coatings of the kind described in DE-OS No. 2,020,153 are also permeable to water vapor.
According to the teaching of Japanese Patent Application No. J 61 009 423, polyethylene glycols may be used as diol components for preparing polyester polyols and subsequently polyurethane elastomers to produce coatings which are permeable to water vapor and have a negligible tendency to well in water.
Segmented polyurethane elastomers obtained from polyethylene glycols are also claimed in European Patent Application No. 52,915.
Other organic hydrophilic components are also added to polyurethanes to produce coatings and composite materials which are permeable to water vapor. In particular, poly-.omega.-methylglutamate may be added to polyurethanes, used as a starting component or grafted onto a polyurethane. DE-OS Nos. 1,922,329 and 1,949,060 and Japanese Patent Application Nos. J 58 057 420 and J 59 036 781 are only a few of the many literature references in which this use is described.
Recently, it is mainly polyurethanes containing the above-mentioned polyethylene glycols as starting components which are of particular technical interest for the field of compact coating compounds permeable to water vapor. These raw materials are inexpensive and widely available. The polyurethanes and polyurethane ureas obtained from them are also known in principle. In contrast to the widely used polyurethane ureas which contain polyesters, polycarbonates or polyethers as macrodiols, the above-mentioned polyurethanes are water-absorbent and permeable to water vapor, and in some cases even swell strongly or dissolve in water.
Hydrophobic polyols are therefore added to the polyethylene glycols which provide the hydrophilic properties. Such mixtures give rise to polyurethanes and polyurethane ureas which have good permeability to water vapor combined with high resistance to the action of liquid water.
Two-component coating systems of ketoxime-blocked prepolymers and cycloaliphatic diamines as cross-linking agents/hardeners prepared with at the most 15% by weight of solvent are claimed in DE-OS No. 2,902,090 (=U.S. Pat. No. 4,248,756). The polyhydroxyl compounds in the blocked prepolymers, for example, polypropylene oxides, may also contain ethylene oxide units.